Actually, a TV continuation could deal with Earth's future after the end of NuBSG - the story implied that despite the warning signs, Earth (or rather, Earth 2) may have the potential for a different outcome to the experiment. The fate of the freed Centurions is also in question, as they sailed away and now 150,000 years have passed. What did they evolve into?

This. If it was up to me, I'd do this. Begin with man being a space-faring civilization again and, at the same time the AI's are coming to full sentience on Earth and the cycle becoming dangerously close to repeating, man meets "cousins" in space, who turn out to be what the Cylon Centurions became. Conflict arises as man learns the true nature of its cousins and our own Cylons reach out to them as well. What side will they take? Will the Cylons in space become divided themselves? Will the cycle repeat? Perhaps the Centurions went back to Earth 1 and terraformed it - or back to the Colonies themselves. The story would actually be about humanity itself coming to grips with its own identity.

You could have it as our Earth gets knackered and the survivors have to travel across the stars to the fabled twelve colonies...

No, just... no.
Too hell with Earth. I'm all Earthed out. I could use an escape that doesn't require a fixed point of reference being Earth itself. It's the same tired story with countless cliches. Everything happens in either New York city or Los Angeles. The Americans save the day, blah blah blah.